The present invention relates to an optical module and a package applied to the optical module used in optical communications, for example.
Fiber gratings form diffraction gratings obtained by causing periodical variations in the refractive index along the length of an optical fiber. For example, fiber gratings are formed by irradiating an optical fiber with an interference pattern of ultraviolet light so as to cause light-induced variations in the refractive index in a core of the optical fiber.
The interference pattern of ultraviolet light may be formed by applying the ultraviolet light through a mask (phase mask) provided with a grating forming pattern. Such a method for forming a grating using the phase mask is called a phase mask method. Further, a holographic method or the like is known which does not use the phase mask to form the interference pattern of ultraviolet.
The fiber gratings have the function of reflecting light with a relatively narrow range of wavelengths with the Bragg reflection wavelength as the center. The Bragg reflection wavelength is determined from the spacing of the diffraction grating and the effective refractive index of the core. Such fiber gratings are used as a single-wavelength filter with excellence in wavelength selection.
In addition, when it is assumed that the Bragg reflection wavelength of a fiber grating is xcex, the effective refractive index is n, and grating pitch is xcex9, there is a relationship therebetween that xcex is equal to 2n xcex9 (xcex=2n xcex9). Since both the effective refractive index n and grating pitch xcex9 have the temperature dependency, it is known that, for example, the Bragg reflection wavelength of a silica-based optical fiber has the temperature dependency of about 0.01 nm/xc2x0 C. to 0.015 mm/xc2x0 C.
The temperature dependency of the Bragg reflection wavelength is so-called positive temperature dependency where the effective refractive index and grating pitch increase as the temperature increases.
Then, in order to compensate for the temperature dependency of the Bragg reflection wavelength, a method has been proposed for forming a temperature-compensating package for fiber gratings using a member with a negative linear expansion coefficient or a member obtained by combining two types of materials with different linear expansion coefficients, and fixing an optical fiber with fiber gratings to the temperature-compensating package.
In addition, thus proposed temperature-compensating package applies a stress in the direction that decreases a length of a fiber-grating formed portion of the optical fiber as the temperature increases, but it is difficult to compress the optical fiber by the stress.
Then, it is designed in the proposed optical module that an optical fiber is fixed to the temperature-compensating package with a predetermined set tensile stress applied to the optical fiber, for example, at room temperature (for example, 25xc2x0 C.), and thereby the tensile stress decreases or becomes zero at high temperature. The optical module is produced by fixing the optical fiber to the temperature-compensating package while applying the set tensile stress to the optical fiber at room temperature.
In this way, in the proposed optical module it is possible to provide the stress applied to the fiber gratings from the temperature-compensating package with negative temperature dependency (it is possible to suppress increases in the pitch of the fiber-grating formed portion as the temperature increases), and therefore, it is possible to compensate for the positive temperature dependency of the refractive index of an optical fiber.
In addition, when the optical fiber provided with fiber gratings is fixed to the temperature-compensating package, a fixing material is used such as a low-melting-point glass, metallic solder or adhesive agent.
With progresses in computerized society, the communication information amount tends to increase dramatically, demanding increases in speed and capacity, which are necessary and indispensable, in optical fiber communications. As an approach to increasing the speed and capacity, a wavelength division multiplexing scheme has been studied that transmits signal light with a plurality of wavelengths that are different from one another at set wavelength intervals, for example, 0.8 nm, using a single optical fiber.
With the study on the wavelength division multiplexing scheme, in recent years, optical components have been required such as OADM (Optical Add Drop Multiplexer) capable of selectively extracting or adding light with a plurality of wavelengths such as four-wave and twenty-wave from/to the transmitted wavelength-multiplexed light.
In order to assemble such optical components, for example, it is considered that a plurality of optical modules are connected each composed of a package accommodating an optical fiber, fixed to the package, with fiber gratings formed therein, and that Bragg reflection wavelengths reflected by fiber gratings of the optical modules are set at respective wavelengths that are different from one another.
However, because such an optical component is formed by connecting a plurality of optical modules, there arise problems that the cost is increased and the optical component is enlarged corresponding to such a configuration. Therefore, an optical module is required that enables a plurality of fiber gratings to be accommodated collectively in a package. However, conventionally, an optical module has not been proposed in which a plurality of fiber gratings are collectively accommodated in a package.
Further, since the wavelength interval is narrow, as described above, for selectively extracting or adding light in an optical component such as OADM, it is required in the optical module composing the optical component that the Bragg reflection wavelength is coincident with a set wavelength accurately.
However, because the Bragg reflection wavelength of a fiber grating depends on tensile, as well as temperature and is affected by contraction, expansion, heat, or the like of the fixing material, it is extremely difficult to make a difference between the Bragg reflection wavelength and set wavelength less than or equal to, for example, 0.1 nm.
An optical module comprising: a comb-shaped package having a plurality of teeth portions and a base portion; and at least one optical fiber having fiber grating, wherein respective fiber-grating formed portions of said at least one optical fiber are placed in corresponding teeth portions and base portion of said package, and said optical fiber is fixed to said base portion and corresponding teeth portion in such manner that said fiber-grating formed portion exists between said base portion and said corresponding teeth portion.